The present invention relates generally to noise reduction, and in particular to a noise reduction circuit which eliminates impulse noise that occurs in broadcast stereophonic composite FM signals using a linear interpolation technique and eliminates noise that occurs as a result of switched separation of received composite FM signal into left- and right-channel signals.
One method currently available for suppressing impulse noise that contaminates audio signals involves reducing the transmission gain or shutting off the transmission path of the signal as long as the noise is present. Another method involves detecting the amplitude of the desired signal on the rising edge of an impulse noise and retaining the detected amplitude in the presence of the impulse noise. While these methods are effective in suppressing impulse noise, the noise-affected portion of the signal is not reconstructed, resulting in unnatural sound. To overcome this problem modern digital audio systems utilize linear interpolation technique to predict the original waveform of the noise-affected portion for the noise-affected portion of the signal. This type of systems requires complicated, expensive circuitry, not suitable for moderate cost equipments.
The aforesaid Copending U.S. Application discloses an impulse noise reduction circuit for FM signals. Received FM signal is demodulated and passed to a first sample-and-hold circuit which tracks the waveform of the signal when no impulse noise is present and samples it in response to a noise-triggered sampling pulse so that the signal is held at the level that occurs immediately prior to the noise. The output of the first sample-and-hold circuit is applied to a feedback circuit which includes a differentiator. The slope ratio of the demodulated signal which occurs immediately prior to the noise is detected by the differentiator whose output is sampled by a second sample-and-hold circuit in response to the sampling pulse, generating a signal as an indication of the slope ratio of the noise-affected portion of the demodulated signal. This signal drives a voltage-controlled bidirectional constant current source to linearly vary the voltage sampled by the first sample-and-hold circuit.
However, one disadvantage inherent in the differentiator is that it tends to accentuate a high frequency component of an input applied thereto since its output is proportional to the rate at which the input signal changes, and the accentuated high frequency component is sampled, causing a distortion of the correction signal which is used to interpolate the noise-affected portion of the signal. This is particularly severe when the noise reduction circuit is employed in an FM receiver adapted to receive stereophonic composite FM signal since the receiver provides switching of the demodulated signal into left- and right-channel signals at a frequency twice the frequency of the 19-kHz pilot signal and this switching presents a high frequency signal to the differentiator.
Another approach that proves useful for suppressing an impulse noise in stereophonic FM signals makes use of a sample-and-hold circuit connected between the output of a demodulator and the input to a separation circuit by which the demodulated signal is switched between left- and right-channel deemphasis circuits. An impulse noise detector is connected to the output of the demodulator to generate a sampling pulse with which the sample-and-hold circuit samples the signal immediately prior to the occurrence of the noise and holds it in the presence of the noise. However, the switching action of the separation circuit chops the sampled signal and produces a noise in one of the deemphasis circuits.